Direct D2D communication in addition to cellular communications in a wireless network has been proposed in the prior art. D2D technology exploits the proximity of the communicating devices to enhance cell throughput, energy efficiency and the achievable data rates. Direct D2D communication between D2D User Equipments (D-UEs) that reuse radio resources of the cellular system is seen as a key enabler for future wireless and mobile communication systems.
In order to increase the spectrum utilization, a cellular network can allow multiple D-UEs to share radio resources with Cellular User Equipment (C-UE), i.e. with User Equipments (UEs) directly communicating with an access point, such as a base station, as long as the D2D transmissions performed between D-UEs are tolerable for the cellular transmissions performed by C-UEs. This network-controlled, a.k.a. network-assisted, way of D2D communication is referred to as underlay, and has the potential to significantly increase spectrum usage, and thus the overall network performance.
Such D2D communication underlying a cellular system operation can be realized via the following four implementations, which represent operation in different modes:    1) Silent Mode: When available radio resources are not sufficient for supporting the D2D traffic, and when resource reuse is not possible, due to interference problems, the D-UES cannot transmit data, and have to stay silent. However, they can continue to operate in cellular mode.    2) Reuse Mode: D-UE devices directly transmit data by reusing radio resources of the cellular network. The resource reuse can be accomplished by using either uplink or downlink resources.    3) Dedicated Mode: The cellular network dedicates a portion of the resources for D-UE devices for their direct communications.    4) Cellular Mode: D2D traffic is relayed through network node(s) in the traditional way.
The dedicated mode and the cellular mode, respectively, simplify the task of interference management. The maximum transmission power can be used in these two modes, in order to provide the best system performance, since D2D links, i.e. communicating D-UE pairs, do not interfere with other cellular users. However, these two modes do not utilize the resources efficiently so as to maximize the overall network throughput.
In contrast, the reuse mode is better in terms of its spectral efficiency. In reuse mode the system performance can also be improved by transmission power optimization. However, explicit and complete knowledge of link characteristics, including interference levels between radio links potentially sharing the same radio resource is needed for efficient implementation of any reuse based algorithm/method. Moreover, mining of such multi-dimensional data is cumbersome and costly with respect to measurement and signaling efforts. Further, if cellular radio resources are reused for facilitating D2D communications, there is a potential risk of mutual interference between existing cellular and established D2D links, and thus overall system performance degradation.
Many state of the art approaches have focused on the co-channel interference induced by D2D operation. However, in these approaches a network node, e.g. base station, needs to know the channel state information (CSI) of all the involved links, which increases the complexity of the system.
Other state of the art approaches proposed geo-location as a substitute for radio location information, where the instantaneous link qualities between the communicating devices can be estimated based on their true geographical locations. Geo-location information can also be exploited, in order to mitigate the Cellular to D2D (C2D), D2D to Cellular (D2C), and inter D2D-to-D2D (inter-D2D) interference. However, issues such as energy efficiency, accuracy and computational overhead decide on the applicability of a particular positioning technique.
In terms of positioning technologies, network-assisted techniques such as Angle of Arrival (AoA) and Time Difference of Arrival (TDoA), are preferred over UE-based techniques, such as Assisted GPS (A-GPS) or Global Navigation Satellite System (GLONASS), since they require less signaling overhead. However, the network-based positioning techniques suffer from higher positioning inaccuracies than the UE-based techniques.
In network-controlled D2D communication, the network decides, whether communication between two UEs is allowed to take place directly (D-Mode) or via the cellular infrastructure (C-Mode), e.g. a base station. In both cases, the same radio resources are used. Hence, there is a potential risk that sub-optimum mode selection decisions may easily lead to severe interference rise and deterioration of already established cellular links. Hence, the network must also use such pre-emptive knowledge in its decision making. This decision can be based on many factors, such as the geo-location and distance between the potential D-UEs, channel and radio conditions, UE capabilities, etc.
The choice of a certain resource allocation strategy has a direct impact on overall efficiency and cell throughput. RA schemes that are able to adapt to varying user distributions and constellations, e.g. more D2D than cellular users, can greatly improve system performance.
An exemplary system of the prior art shown in FIG. 17 comprises an LTE evolved NodeB (eNB) 1700 located at a cell center, and a number of randomly placed users (C-UEs 1701 and D-UEs 1702), where users may be distributed in a uniform or non-uniform manner. D2D communication takes place in the LTE band using either TDD or FDD, where cellular resources, i.e. Physical Resource Blocks (PRBs) are reused.
Several possible interference scenarios are illustrated in FIG. 17. Due to D2D communications operating in parallel to cellular communications and reusing cellular radio resources, several new and complex interference issues arise, such as C2D, D2C, and inter-D2D interference. Thereby, a link corresponds to a communication link, over which desired information, data and/or control information, is sent bidirectional or unidirectional between the involved entities. Interference is the undesired signal, which can reduce signal-to-interference-plus-noise ratio (SINR) of the desired signal, and thus can cause performance degradation, e.g., in terms of throughput. Accordingly, the term D2D link refers to a D2D communication link, via which the desired signal is transmitted and/or received between a D-UE pair. The term cellular link refers to a cellular communication, via which the desired signal is transmitted and/or received between a C-UE and a base station, e.g. an eNB as shown in FIG. 17. The term C2D interference refers to the disturbance of a D2D communication caused by the undesired signal of a cellular communication. The term D2C interference refers to the disturbance of a cellular communication caused by the undesired signal of a D2D communication, e.g. shown as “interference to cellular eNB” in FIG. 17. The term inter-D2D interference refers to the disturbance of a D2D communication caused by the undesired signal of another D-UE pair.
Therefore, resource management for D2D communication underlying a cellular infrastructure is particularly challenging, since intra-cell and inter-cell interference need to be managed between the cellular and D2D links.
Belleschi et al. “Performance analysis of a distributed resource allocation scheme for D2D communications”, IEEE GLOBECOM Workshops, 2011, presents a framework for joint optimization of mode selection, resource assignment and power allocation. However, optimum performance can only be achieved, if full knowledge on C2D, D2C, and inter-D2D channel gains is available.
Nam et al. “Location-based resource allocation for OFDMA cognitive radio systems”, Proceedings of the Fifth International IEEE Conference on Cognitive Radio Oriented Wireless Networks & Communications (CROWNCOM), 2010, introduces a RA algorithm for OFDMA-based cognitive radio systems, which utilized location information of primary and secondary users instead of the CSI of the interference link. However, accurate positioning technique (therein, GPS) is assumed, in order to remove the CSI dependency for RA. Moreover, the interference is estimated in the probabilistic sense (log-normal shadowing) by considering the distance between the transmitting UE (Tx UE) and receiving UE (Rx UE), thereby also requiring precise channel models.
Further, distance dependent RA schemes are known from the prior art. For example, in one scheme the network assigns a certain D2D link the PRB of a C-UE, which is at a distance L>Lmin, wherein Lmin is a pre-selected distance constraint, in order to control the interference from the selected C-UE to the D-UE using the same PRB. If the D2D transmission is envisioned to be in cellular Uplink (UL), the D2C interference is not effectively eliminated in this case, since the distance between the base station (BS) and the transmitting D-UE is not considered for the RA. However, the C2D interference can be mitigated by optimal selection of the distance Lmin. One major disadvantage of distance-based RA schemes is that for optimum RA decisions, accurate geo-positioning is required, which results in increased signaling overhead and processing delays.